1. Field of the Invention
The present invention relates generally to apparatus for tensioning web wrapping material, and more particularly, to method and apparatus for automatically threading a web of wrapping material around a plurality of rollers.
2. Description of the Prior Art
In a number of important industrial applications, a web is advanced from a feedstock roll to an operating station over a plurality of rollers, one or more of which are driven. In some installations the web may be threaded around the rollers by hand. Manual threading is difficult where access to the rollers is limited. Moreover, manually threading the web is time consumming and presents a risk of personal injury by exposing the operator to power driven components. Some web material, for example stretchable film of the type used for unitizing pallet loads, is very flexible and therefore relatively difficult to manipulate around the rollers.
Recent improvements in film web pretensioners for unitizing pallet loads utilize one or more driven rollers. In a typical dual drive arrangement, the film web undergoes stretching by the tensioner assembly as it is dispensed from a feedstock roll. The tensioner assembly includes two driven rollers which pull the film web from the feedstock roll which is coupled to a brake for maintaining a drag force on the feedstock roll. The film web is stretched in a first stage between the feedstock roll and the first drive roller. A second stage of stretching occurs between the first driven roller and the second driven roller. Finally, the film web undergoes a third stage of stretching between the second driven roller and the pallet load.
The foregoing web pretensioner and dispenser assembly is utilized on high speed conveyor lines in which pallet loads are unitized by stretchable film web on a mass production basis. Although the pretensioner apparatus greatly increases the coverage of a roll of film, it is necessary to insert a new roll of film web material from time-to-time as each roll is used up, or when changing over to a film material of a different thickness or grade for a new wrapping operation. For high speed, mass production operations, downtime is very expensive. Therefore, it is desirable to minimize the downtime associated with the change-over of the film feedstock roll. Present procedures which involve manual threading of the film web are time consumming because of the limited access to the drive rollers, and also expose the operator to the risk of personal injury.
A further problem relating to the threading of a stretchable film web around two or more drive rollers is that the film web has a tendency to cling to the surface of the drive rollers. Moreover, the drive rollers usually are provided with a jacket of rubber or other material which enhances the clinging effect. Because of this, the film becomes wrinkled and misaligned with the feedstock roll as it is threaded around the drive rollers.